Conventional systems exist that are designed to interpret user motion as input to a system. For example, proximity detectors are commonly used to activate motion lights. Further, touch screen systems, such as track pads, mobile telephone interfaces, information kiosks, and various other computing devices can be configured to use capacitive sensors to identify a location on the touch screen corresponding to the user's interaction with the touch screen via a stylus or finger. In such systems, changes in capacitance can be interpreted to determine a contact location and/or to detect motion.
Another technique for detecting motion can include optical sensors that are configured to capture optical input on a pixel-by-pixel basis and to identify motion by analyzing changes in the pixel data. Optical sensor circuitry typically requires a transparent window through which reflected light can be received. However, transparent packaging of an integrated circuit die is more expensive than typical opaque packaging. Further, such optical sensor circuitry may have stringent pad requirements and low pad density for coupling to other circuitry.